Support the "version" property on volumes via the zfs_prop_get_int() API

Support the "version" property on volumes via the zfs_prop_get_int() API

As of this commit, volumes do not possess the version property in
any existing OpenZFS implementation.  The zpool upgrade code, however,
uses zfs_prop_get_int() to fetch the version property of all children in
a pool.  The semantics of the function, however, demand that it only be
used for known valid properties so it returns a garbage value for volumes.

This patch causes the version of a volume to appear to callers using
the zfs_prop_get_int() API to be that of the default ZPL version for
the implementation.  In the future, should volumes gain the property,
its actual value will be used.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fixes #3313